I have had no experience of that specific product, but it seems to be a good quality cavity insulation material. I understand that it should be fitted 'snugly' inside the cavity, butt-jointed tightly possibly with taped joins, and installed around corners in a specific way - all to ensure the max U value over the whole wall.

Make sure your builder appreciates these installation points, but there should be no issues - and you should have a cosy extension.

we have a builder X who had bought this at cut-price from another building place (i think these were leftovers). we never asked too much details of it. recently another builder Y came to property to quote our extension and he had some reservations

Builder Y comments:
The insulation is thick for the cavity
It should de maximum of 50mm thick celotex or kingspan and leaves 50mm for ventilation of the cavity .
You can use 100mm earthwool insulation
Which I think is better for cavity.
And I don’t think you can use that on roof because it doesn’t have the foil on doth sides .

So my concerns are if builder Y says true, and if thats the case, then we shouldnt use it?

My other concerns are, as the builder X bought this and a lot of timber for our extension work a year ago, though we our extension only starts in few weeks, they were left in our garden for year exposed to all types of weather. Would this be a problem/has its risks? (they were not covered)

I would personally rather this form of 'rigid' insulation in a cavity over earthwool.

What thickness are these batts? My only 'concern' about builder 1's specs is why there would also need to be 40mm insulated plasterboard required if 100mm batts were used in the cavity; wouldn't it make more sense to use lightweight insulated blocks for your inner skin rather than dense and then forget the 40mm TLB? But I should emphasise that I don't know what current requirements are.

Builder 2 seems to know less about these insulation batts than I do. He says he'd want a ventilation gap in the cavity - whereas he proposes to stuff it with earthwool. What's more, I think the idea of these rigid blocks is that there is no gap left - the second skin should be assembled snugly against it.

Whether these same batts can be used in roofs I simply cannot comment on.

Oh, I wouldn't worry about these batts being left outside in the rain, but I would be concerned about the timber - if they have been soaked over many months, expect lots of creaking, groaning a surface cracks in your nicely applied plaster skim coat. That might not happen, but the wetter the timber the more it'll move after being fitted inside a dry environment.

I will come back with more questions .... builder 2 certainly insists he wouldnt use it. He was saying the wall thickness increases too, like you have the block 50mm, then the cavity, which is 100mm, space 50mm (for ventilation- he insists ventilation helps damp proofing) and then 50mm block = 350mm and to find lintle 350mm is not possible.

If the/a builder want to deviate from what's been specified, presumably on cost grounds, why not see what the arcitect has to say. It might be that the architect has made particular specifications for particular reasons. They may have already liaised with, or have local knowledge of what the local building control folk are happy with.

I will come back with more questions .... builder 2 certainly insists he wouldnt use it. He was saying the wall thickness increases too, like you have the block 50mm, then the cavity, which is 100mm, space 50mm (for ventilation- he insists ventilation helps damp proofing) and then 50mm block = 350mm and to find lintle 350mm is not possible.

Click to expand...

Sammi is indisposed, but she asked me to pass on this info: builder 2 is an ejit.

Well, at least as far as this product is concerned. A quick Google seems to suggest that this Cavitytherm rigid insulation is terrific stuff and even has a layer specially designed to repel any wind-driven rain that might penetrate your outer skin. It does NOT require or benefit from an air gap, hence your builder being silly about this. And about it increasing your wall thickness. And about 100mm rockwool being preferable.

I'm not suggesting you don't use B2, because he might well be very competent in other areas, but his homework should be to look at this product and then come back to you apologetic, and to never ever again suggest using rockwool in a cavity over this stuff. And, if he were daft enough to stuff 100m roockwool in to your cavity, where's your air gap there?

Sheeesh. You have a top-notch product at your disposal and you are toying with going back to the 19th century? Sheeeeesh...

1. Product is confirmed (if 100mm) as full cell cavity wall insulation - obe if their premium products.
2. It Can be used for external cavity walls either with blocks (standard blocks) or one side blocks n other side bricks. Plastic side to be faced to external side.

3. Re: gap. Small ridges in plastic side are about 5mm. Board itself is actually 95 cm in thickness. So that's how they achieve circulation gap. Asked our area and confirmed these related to concern on damp. Should be fine.
(Their site talk about I value .12...)
4. Confirmed this type Can NOT be used in roof or floor. Only for cavity wall.

5. Re: left out in rain..and for months : said they should have been covered. If plastic cover damaged done may have compromised integrity. So might not be able to use all. Might be on if left to dry property. Hard to comment etc..but if we send pictures they can look n give a more reliable answer.